Yes, yes, yes!

This bugs me quite a lot. Consider this: http://try.haxe.org/#e0EC4

Chrome: 

Test.hx:29: 0.0280001163482666s
Test.hx:30: 0.0019998550415039062s

Firefox:

Test.hx:29: 1.071000099182129s //yep, enums are veeeery sloooow on firefox
Test.hx:30: 0.002000093460083008s

IE11:
Test.hx:29: 0.06400012969970703s
Test.hx:30: 0.003000020980834961s

That"s over an order of magnitude.

I can live with having to optimize bottlenecks to not use enums, but if it can be fast out of the box, that"d be great.

It looks nice, but I have no idea how to implement that.

Could we somehow inline enum constructors similar to what we do with array and object declarations?

Perhaps, but that"s not really the issue here.

Could someone post a practical example of this which makes sense in the real world?

@Simn how about this?

function onKeyDown(key) {
  if(key == "up") walk(Up);
  if(key == "down") walk(Down);
  if(key == "left") walk(Left);
  if(key == "right") walk(Right);
}

function walk(direction:Direction) switch direction {
  case Up: player.y --;
  case Down: player.y ++;
  case Left: player.x --;
  case Right: player.x ++;
}

enum Direction {
  Up; Down; Left; Right;
}

This example is more or less extracted and modified from here. See how FlxWeaponFireMode is basically used only in one function, so I think I agree with @nadako that this is quite related to inline enum declaration.

That is not the same issue and already produces good output if you inline walk.

here is another example:

import haxe.ds.Option;

abstract Maybe<T>(Null<T>) {
	public function new (x:Null<T>) this = x;

	@:analyzer(optimize) public inline function match ():haxe.ds.Option<T> {
		return this != null ? Some(this) : None;
	}
}

class Test {
	@:analyzer(optimize) static function main () {
		var m = new Maybe(null);
		var x = switch (m.match()) {
			case None: 0;
			case Some(x): x;
		}

		trace(x);
	}
}

becomes

var x;
var _g = m != null?haxe_ds_Option.Some(m):haxe_ds_Option.None;
switch(_g[1]) {
case 0:
	x = _g[2];
	break;
case 1:
	x = 0;
	break;
}

instead of

var x = (m != null ? m : 0);